Cost Analysis

2.5.1.1 Factors

In this visible communication system’s design, both the receiver and the transmitter are interfaced with a computer using a microprocessor. When it came to deciding which processor to use for the design, there were a few factors to consider. Since the processor is meant to connect directly to the computer, the first factor that was considered was USB connectivity. Having a USB connection between the computer and the processor would allow the system to transfer information to and from the computer. By not having this functionality, it would not be possible to meet the goal of interfacing the communication system with the computer. To analyze if a processor met the requirement, it was given a value of five if it had USB connectivity, or a value of zero if it did not. The following table represents the breakdown of how the points were assigned for USB connectivity.

USB Connectivity Value

No 0

Yes 5

Table 6: USB Connectivity Value Assignments

Another factor that is as crucial as the USB connectivity is the processor’s sampling rate. One of the design goals was to have the system’s transfer rate be in the megabits per second (Mbps) range. To meet this design, the microprocessor must have an analog to digital converter, or ADC, that can produce at least a million samples per second. The reason for this is due to the Nyquist rate, which states that a signal must be sampled at a frequency that is twice the signal’s highest frequency content in order to keep the input signal intact. It is also possible to use a sampling rate that is exactly the same as the signals input and not cause interference. The following table ranks a few sampling rates into a value system ranking from zero to five.

Sampling Rate Value

>1000ns 0 800-1000ns 1 600-800ns 2 400-600ns 3 200-400ns 4 0-200ns 5

Table 7: Sampling Rate Value Assignment

As mentioned before, since it is preferred to have a transmission rate in the mbps range, the ADC on the processor cannot have a sampling rate that is less than 1000 ns, which corresponds to 1

35 MHz. If the processor had a sampling rate that was slower than 1000 ns, it was not considered and given a zero. As the sampling rate decreases the corresponding value increases, which allows the system to operate at a higher rate.

The next factor that was considered was price. When deciding which processor to choose, the price of the processor and its development board were compared. Due to the project budget being set to roughly $300, the team determined that any board processor combo that was over $100 exceeded the budget and therefore were not considered. The following table describes the price breakdown for the processors and the corresponding values for them.

Cost Value $100+ 0 $60-$80 1 $40-$60 2 $20-$40 3 $10-$20 4 < $10 5

Table 8: Price Value Assignment

The last factor that was considered for choosing the processor was its power supply. Since the visible light communication system was intended to be powered through batteries, there was a limited amount of power supply. Because of this, if the processor ran on an average power that exceeded 3.3V it was given a value of zero, and if the average was at or below 3.3V, the value was set to five. The following table represents the value breakdown for the processor’s power.

Power Value

> 3.3V 0

< 3.3V 5

Table 9: Power Value Assignment

2.5.1.2 Processors

When it came to choosing which processor is right for the design, the choices were narrowed down to four processors. The processors are all manufactured by Texas Instruments, and the manufacture names are: MSP430BT5190, MSP430F5529, TMDX28069USB, and LAUNCHXL-F28027. The following table represents each processor’s sampling rate, price, power supply, and USB connectivity.

Processor MSP430BT5190 MSP430F5529 TMDX28069USB LAUCHXL-F28027

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USB Connectivity No Yes Yes Yes

Supply Voltage 3.3V 3.3V 3.3V 3.3V

Price $101.00 $15.24 $40.50 $17.70

Table 10: Processor Information

To get a better understanding of how the processors compare to each other, the processors’ information was converted to numerical values using the value assignments. Each factor was also assigned a weight based on importance. Since the team determined that sampling rate and USB connectivity were equally important, they were assigned a weight of 50. The processors’ price was also important but not as significant as the other factors so its weight was assigned to 30. The power was the least important because all processors had the same power supply; thus, the weight was set to 5.

To get the overall value for the processors, each processor’s factor value was multiplied by the factor weight and then added up to achieve a total weight value. The following table represents the results in determining which processor to use for the design.

Weight 30 50 50 5

Processor Price Sampling Rate USB Connectivity Power Total

MSP430BT5190 0 1 0 5 75

MSP430F5529 4 1 5 5 445

TMDX28069USB 2 4 5 5 535

LAUCHXL-F28027 4 4 5 5 595

Table 11: Processor Value Analysis

Based on the results shown in the above table, the LAUCHXL-F28027 and the TMDX28069USB processors were better than the other options. Both systems use chips that come from the same line of microprocessors but the development boards are different. We decided to go with the LAUCHXL-F28027 as it had the largest total and that it was cheaper than the TMDX28069USB.

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Figure 13: LAUCHXL-F28027 development board

After testing and debugging the LAUNCHXL-F28027 for several weeks on the receiver side, it was determined that the onboard ADC was not taking samples in a predictable manner. It was imperative that we could control the sampling rate in order for the receiver module to decode a message correctly. To compensate for this problem, the decision was made to switch to an actual microcontroller whose family the team was more familiar with, rather than to continue using the digital signal processing chip. Although our value analysis indicated that the TMDX28069USB would have been our second choice, we decided on the MSP430F5529 due to our familiarity and prior experience with the MSP430 microcontroller family.

For our purposes, the main significant difference between the MSP430F5529 and the LAUNCHXL-F28027 is the sampling rate of the ADC. The MSP430F5529 microcontroller lists a sampling rate of about 200 kbps, significantly less than that of the LAUNCHXL-F28027 sampling rate of 4.6 Mbps. However, due to an adjustment in goals outlined in Chapter 4, this high sampling rate was no longer necessary. Thus, we opted for a slower sampling ADC that had predictable functionality over a faster sampling ADC that had unpredictable behavior.